Polymer Brushes via Surface-Initiated Polymerization catalysts in the final polymer brushes might have undesirable consequences for applications, such as in the biomedical or electronic industry. However, some methods, in particular A(R)GET ATRP, have been developed that allow to reduce the amount of copper to the level of a few ppm. 72 Surface-Initiated Reversible-Addition Fragmentation Chain Transfer (SI-RAFT) PolymerizationIn contrast to ATRP, where the equilibrium between the dormant and active, propagating chains is based on reversible termination, reversible-addition fragmentation chain transfer (RAFT) polymerization is based on reversible chain transfer. [114][115][116] A distinct advantage of RAFT polymerization is its relative simplicity and versatility, since conventional free radical polymerizations can be readily converted into a RAFT process by adding an appropriate RAFT agent, such as a dithioester, dithiocarbamate, or trithiocarbonate compound, while other reaction parameters, such as monomer, initiator, solvent, and temperature, can be kept constant. RAFT polymerization has also been successfully used to prepare polymer brushes via surface-initiated polymerization. SI-RAFT can be performed using two different strategies, which use either surface-immobilized conventional free radical initiators or surface-immobilized RAFT agents (Scheme 2). These two different strategies will be discussed in more detail in the following paragraphs.

The reaction of the aminophenols 2-(CH2L)-6-R-C6H3OH (R = Ph, L = NMe2, 1a; R = t Bu, L = NMe2, 1b; R = t Bu, L = NC4H8, 1c; R = t Bu, L = NC5H10, 1d) with 1 equiv of AlMe3 affords the corresponding dimethyl Al complexes {2-(CH2L)-6-R-C6H3O}AlMe2 (R = Ph, L = NMe2, 2a; R = t Bu, L = NMe2, 2b; R = t Bu, L = NC4H8, 2c; R = t Bu, L = NC5H10, 2d) in high yields. Compounds 2a−d appear to be monomeric, on the basis of X-ray analysis for 2b,d and NMR data for 2a−d, and are stable in the presence of THF. {2-(CH2NMe2)-6-Ph-C6H3O}AlMe2 (2a) cleanly reacts with B(C6F5)3 to yield the dinuclear cationic Al species 3a +. X-ray diffraction analysis shows that the cation 3a + can be seen as an adduct of the three-coordinate cation {2-(CH2NMe2)-6-Ph-C6H3O}AlMe+ and the neutral precursor 2a, in which the two Al centers are connected via a μ2-O aminophenolate bridge. Similarly, the reaction of Al dimethyl complexes 2b−d with B(C6F5)3 yields dinuclear cationic Al species 3b−d +/3b‘−d‘+ (3b +/3b‘+ in a 1/1 ratio; 3c,d +/3c‘,d‘+ in a 3/1 ratio, respectively) as diastereomeric mixtures. Cations 3b−d +/3b‘−d‘+ adopt a structure similar to that of cation 3a +, as determined by X-ray crystallography analysis for 3b‘+ and 2D NMR studies for 3a + and 3c,d +/3c‘,d‘+. All the formed dinuclear cations are quite stable and robust in solution, and no fluxional behavior for any of them was observed up to 80 °C in C6D5Br. Cations 3b−d +/3b‘−d‘+ react with a Lewis base such as THF to afford the corresponding THF adduct cation 4b−d + along with 1 equiv of the corresponding neutral precursor 2b−d. In contrast, 3a + reacts with THF to yield unidentified species. 3a + and 3c,d +/3c‘,d‘+ are inactive in ethylene polymerization, but cations 3b−d +/3b‘−d‘+ polymerize PO with moderate activity to yield low-molecular-weight PPO.